There has been considered application of a nitride semiconductor to a high-withstand-voltage and high-power semiconductor device by utilizing its characteristics such as a high saturation electron velocity and a wide band gap. For example, GaN which is a nitride semiconductor has a band gap of 3.4 eV, which is larger than a band gap (1.1 eV) of Si and a band gap (1.4 eV) of GaAs, and GaN has high breakdown electric field intensity. This makes GaN very promising as a material of a semiconductor device for power supply which obtains high voltage operation and high power.
As a semiconductor device using a nitride semiconductor, many reports on a field effect transistor, particularly a high electron mobility transistor (HEMT) have been made. For example, in a GaN-based HEMT (GaN-HEMT), an AlGaN/GaN HEMT using GaN as an electron transit layer and AlGaN as an electron supply layer has been receiving attention. In the AlGaN/GaN HEMT, distortion caused by a lattice constant difference between GaN and AlGaN occurs in AlGaN. By piezoelectric polarization caused by the above and spontaneous polarization of AlGaN, high-concentration two-dimensional electron gas (2DEG) is obtained. Therefore, the AlGaN/GaN HEMT is expected as a high-withstand-voltage power device for a high-efficiency switch element, an electric vehicle, or the like.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2007-200975
[Patent Document 2] Japanese Laid-open Patent Publication No. 2005-191477
FIG. 9 is a schematic cross-sectional view schematically illustrating a configuration of a compound semiconductor stacked structure in a conventional AlGaN/GaN HEMT.
A compound semiconductor stacked structure 102 is formed on, for example, a SiC substrate 101. The compound semiconductor stacked structure 102 is constituted by stacking a non-illustrated nucleus formation layer made of AlN or the like, a buffer layer 102a made of AlGaN or the like, an electron transit layer 102b made of GaN, an electron supply layer 102c made of AlGaN or the like, and a cap layer 102d made of GaN or the like. To the electron supply layer 102c and the cap layer 102d, an n-type impurity (n-type dopant) of Si or the like is added. The addition of the n-type dopant aims at an increase in two-dimensional electron gas (2DEG) or the like in the electron supply layer 102c and at improvement in an ohmic characteristic between the cap layer 102d and an electrode formed thereon and relaxation of an electric field concentration or the like in the cap layer 102d. Achieving these aims is intended for higher power and higher efficiency of the AlGaN/GaN HEMT.
FIG. 10A to FIG. 10C are schematic views for explaining problems in the conventional AlGaN/GaN HEMT.
In growth of a GaN crystal by using a metal organic vapor phase epitaxy (MOVPE) method, for the purpose of preventing carbon which easily becomes a disturbing factor in device performance from mixing, the growth is generally performed at a pressure of, for example, 200 mbar or more. On the other hand, in layers containing Al such as the electron supply layer or the like, growth is to be performed under relatively low pressures in order to suppress a vapor phase reaction before the growth (FIG. 10A).
When the compound semiconductor stacked structure 102 illustrated in FIG. 9 is formed, a growth condition is usually changed for the purpose of enhancing film quality of the cap layer 102d after forming the electron supply layer 102c containing Al (FIG. 10B). As a result, switching time (growth interruption time) during which the pressure is increased to become stable occurs. However, when growth interruption is performed under high temperatures, desorption of gallium and nitrogen which are constituent elements of the electron supply layer 102c occurs and the n-type dopant is concentrated on an uppermost surface of the electron supply layer 102c (FIG. 10C). Excessive existence of the n-type dopant causes a defect in a film. Accordingly, the AlGaN/GaN HEMT using a crystal structure such as the compound semiconductor stacked structure 102 has problems that a threshold shift referred to as collapse, a withstand-voltage deterioration starting from the defect, and a deterioration of a leak characteristic occur to greatly impair design performance and reliability.